This invention relates to a sheet suitable for preparing transparencies, more particularly, sheet suitable for preparing transparencies by means of various types of ink depositing devices, e.g. ink jet printers and pen plotters.
Ink depositing devices have been developed to serve as means for recording output for computers and the like. Ink jet printers operate by ejecting droplets of ink through a nozzle onto the surface of an appropriate recording medium. Pen plotters operate by writing directly on the surface of an appropriate recording medium, using a pen consisting of a bundle of capillary tubes in contact with an ink reservoir.
Despite the differences in principle of operation between pen plotters and ink jet printers, the properties required of the inks for the two devices are similar. In both devices, the ink must pass through small openings and be exposed to the open air for long periods of time prior to imaging, because it frequently occurs that the printing or plotting device, i.e. nozzle or pen, will be left uncovered while the machine is not in operation, thereby allowing the ink solvent to evaporate. Consequently, the inks must be of low viscosity and must exhibit low evaporation rates at room temperature. Because both types of devices are expected to be used in an office environment, the inks must be of low toxicity, must not produce offensive odors, and must present no fire hazard. An additional requirement arises in the case of a particular type of ink jet printer, known as the "continuous" ink jet printer. This device depends for its operation upon deflecting the ejected ink droplets by means of an electrostatic field. In order to operate in this manner, the ink must be electrically conductive. It has been found by many ink formulators that vehicles comprising water and other water miscible solvents are very useful for meeting these requirements. Even in cases where water is not used, it has been the experience of ink formulators that the best solvents for meeting the above requirements are water miscible. The class of solvents which includes water and water-miscible solvents will hereinafter be referred to as "hydrophilic" solvents.
The primary requirements for ink-receptive sheets which are to be used in the preparation of transparencies are that the imaging material, i.e. the ink, dry quickly thereon and give an acceptable appearance upon projection. Although quick drying is desirable, it is not desirable that the ink solvent evaporate too quickly. If the ink solvents were formulated so as to evaporate extremely quickly, the ink could form a solid mass in the ink-jet orifice or in the pen capillaries, thereby preventing further operation until the apparatus can be cleaned or otherwise repaired. It has been the experience of those who design and build ink jet printers and pen plotters that orifice clogging can be very troublesome and will almost certainly occur if the ink is not formulated so as to avoid excessive evaporation of solvent.
It follows that drying of the printed image should preferably occur by some means other than evaporation. In the case of paper, the absorption of ink into the pores of the paper provides a good means of drying. For non-porous, transparent polymeric sheets, some other means of drying must be used. The quality of print appearance is determined by a number of features, chief among them being color intensity and uniformity, edge smoothness and sharpness, or "acuity", and absence of scratches, fingerprints, or other blemishes in both the image and background areas.
It has been found that color uniformity and intensity are affected by a property called "spread". When ink is deposited onto the imaging medium, it is desirable that it spread over the surface slightly, so that if a solid area is being filled in, the ink being deposited will meet the already deposited portion of the image, so as to form a uniform coloration. On the other hand, it is desired that the ink not spread to such an extent that the edge of the imaged area loses sharpness, or that one color "bleeds" into another. The terms "dot spread" for ink jet printers and "line spread" for pen plotters represent important properties of ink imaging systems.
Another feature of image quality which is of importance is clarity. Some loss of clarity results from haze inherent in the backing or in the coating thereon. Another detriment to clarity results from surface damage, in the form, for example, of scratches or fingerprints. It has been found that ink-receptive layers which are receptive to hydrophilic inks are often susceptible to fingerprints, especially from moist fingers, so that the requirements of good ink receptivity and surface durability can come into conflict. In addition, it has also been found that ink-receptive surface properties important for good drying, high image quality, and good surface durability are affected by ambient relative humidity. The most desirable ink-receptive surface is one which is hydrophilic, i.e., it is capable of absorbing large amounts of water or water-miscible solvents, but not hygroscopic, i.e., it does not absorb excessive quantities of moisture from the air.
U.S. Pat. No. 4,371,582 discloses a coating for ink jet recording sheets which can provide images having good water resistance and high image optical density. The coating disclosed therein is formulated for use with paper substrates. It provides improved image density and permanence by mordanting the dye contained in the ink. It is assumed that the water or other liquid vehicles in the ink will be removed from the image receptive surface by capillary diffusion into the paper substrate, or by some other means. This invention therefore does not address one of the major problems of ink jet printing onto film, namely how to remove the liquid vehicle from the image in order to effect drying.
United Kingdom Patent No. 2050866 discloses a water soluble or swellable polymer as a coating for paper, cloth, plastic film, metal, and glass sheet. The preferred polymer is poly-N-vinyl pyrrolidone; a high concentration of silica and swellable polymer can be added to help imbibe solvent. U.S. Pat. No. 4,547,405 discloses an ink jet recording sheet suitable for making transparencies for overhead projection. The sheet has a layer comprising 5-100% by weight of a coalesced block copolymer latex of polyvinyl alcohol with polyvinyl (benzyl ammonium chloride) and 0-95% by weight of a water soluble polymer selected from the group consisting of polyvinyl alcohol, poly-N-vinyl pyrrolidone and copolymers thereof.
These ink-receptive coatings generally suffer from high moisture sensitivity. They are readily marked by finger contact, are rendered tacky at humidities above 60%, and they stick to the imaging device when fed therethrough.